The invention relates to a method for laser machining using a laser beam to correct a spot area that deviates from an ideal shape, especially a shape which is symmetrical around a given center point. The invention further relates to an apparatus for carrying out this method.
In the machining of workpieces using a focused laser beam, the spot geometry in the focal plane on the workpiece is decisive for the machining quality in practice. If, as frequently occurs in practice, this spot area deviates from an ideal circular shape (e.g., if the laser spot has an oval shape due to different optical aberrations of the laser source and/or the focusing optics), then the resulting machined surface depends on the direction of advancement because of the different dimensions of the spot area of the laser beam in the direction of the two main axes. For instance, an oval laser spot that is guided along a circular trajectory leads to an oval machining zone and thus to unsatisfactory machining results.
Due to the continuously increasing requirements to be met by laser machining, particularly in view of miniaturization, the influences of errors caused by the oval spot area must be corrected. This involves substantial costs, however.
For instance, the deviation caused by the spot area can be compensated through correction values in the control program to be able to adjust, for instance, a corrected trajectory as a function of the direction of advancement of the laser spot. Such a solution requires substantial control complexity, however, especially since the occurring spot area error can prove to be different in each laser device. As a result, the correction values must first be determined and must be correspondingly taken into account with each change in the control program.
It is also feasible to use an optical element to make the laser beam rotatable relative to the workpiece, such that the position of the main axes of the spot area can be kept constant relative to the advance direction. This solution has the drawback, however, that it requires mechanical actuators, which in practice significantly slow down the machining rate even if only one optical element disposed in the beam path is moved for the rotary motion of the laser beam.
It is an object of the invention to provide a method that will bring about a simple correction or point symmetry of the spot area of a laser machining device.
Another object of the invention is to provide a laser spot correcting method which can be performed without any mechanical actuators or changes in the control program.
It is also an object of the invention to provide an apparatus for carrying out the method of the invention.
These and other objects are achieved in accordance with the present invention by providing a method for laser machining by directing a laser beam against a workpiece to form a laser spot area, wherein an initial laser spot area formed by the laser which deviates from an ideal shape symmetrical about a point, is corrected; said method comprising superimposing a second laser spot area on the initial laser spot area, wherein said second laser spot area is rotated about its optical axis relative to the initial laser spot area by splitting the laser beam into first and second laser beams that form the initial and second spot areas, respectively; reflecting the second laser beam from a reflector which is inclined relative to main axes (X, Y) of the initial laser beam and which has a reflecting surface oriented such that a line normal to said surface forms an angle between 0 and 90xc2x0 with the main axes (X, Y) of the laser beam; and thereafter recombining the second laser beam with the initial laser beam to form a common laser beam of corrected shape.
In accordance with a further aspect of the invention, the objects are achieved by providing an apparatus for use in laser machining to correct a spot area of an initial laser beam that deviates from an ideal shape symmetrical about a point, said apparatus comprising a reflector which is inclined relative to main axes (X, Y) of the initial laser beam and which has a surface such that a line normal to said surface forms an angle between 0 to 90xc2x0 with said main axes; a beam splitter for splitting off a second laser beam from the initial laser beam and directing the second laser beam toward said reflector, whereby said second laser beam is rotated relative to the optical axis of the initial laser beam; and a combining element for recombining the rotated second laser beam with the initial laser beam to produce a common laser beam which forms a corrected laser spot area.